Stretching machines are known, for example for stretching a high voltage electric line, a railway line or other, in general aerial lines, which provide the so-called “braked” stretching of cables by means of multiple capstan machines from which the cables to be stretched or respectively to be recovered, are unwound, or onto which they are respectively wound.
In particular, said stretching devices provide a machine with a winch function configured to recover a cable, or a steel-wire rope, with a drawing function, by winding the latter on a respective pair of collection capstans. Such stretching devices, on the other side of the line to be stretched, comprise a machine with the function of a brake, configured to control the unwinding tension of the cable or cables to be stretched, by winding the cable on the respective pair of unwinding capstans where the cable is put under tension, for example, by the action of the winch machine. In particular, the brake machine exerts a controlled tension on the cables in the stretching step in order to guarantee they can remain suspended to suitable guide devices provided on intermediate supports, and that they are disposed according to the configuration of the suspended catenary wire with controlled inflection. In this way the cables do not slide on the ground and remain distant from possible obstacles present under the cables, such as for example road or railway crossings, intersections with other electric lines etc.
It is also known to use aerial means, such as for example a helicopter, to draw a so-called pilot cable from the brake machine to the winch machine, or, sometimes, to draw the final conductor directly from the brake machine to the destination zone without using the winch machine.
The stretching device, also known as braked stretching device, can also be applied for stretching an electric catenary wire for railway traction. In this case, the brake machine is mounted on a traveling wagon, whether it is self-propelled or drawn and, thanks to the movement of the wagon, the cable is released with a controlled tension through the action of the brake.
It is also known to equip traditional brake machines with a mechanical safety device with automatic drive, also known as negative brake, the purpose of which is to hold the conductor under tension both when the machine is stopped, that is, when the safety device is not powered, and also in emergency situations, for example due to a breakdown in the actuation device.
The negative brake is normally associated with one or more distributor valves that control the activation thereof.
There are also solutions in the state of the art in which the brake machine is provided with a maximum pressure valve, commanded electrically, that is, with a proportional electrical command, which allows to selectively and precisely regulate the working pressure in the device, and consequently the final tension in the stretching cable.
However, when there is a break-down in the electrical drive, the maximum pressure valve is no longer able to control the working pressure in the device, and dangerous conditions may arise.
In particular, if the electrical drive signal to the maximum pressure valve fails, the valve opens completely, discharging the whole circuit controlled, and since the negative brake is de-activated, a loss of tension occurs in the cable, with the consequent and connected risks.
Furthermore, if the distributor valve that powers the negative brake is the unstable type, also known as monostable, that is, the type that when the activation command ceases returns autonomously to its initial functioning condition, this closes the negative brake and returns it to the blocked condition. The blockage of the negative brake entails a recoil on the cable under tension and/or on the mean that is drawing the cable.
Known winch machines for braked stretching are normally equipped with devices to limit and control the overload which, if used correctly, prevent overload situations along the path of the cables stretched.
When stretching is performed with a mean other than winch machines, and in order to carry out the traction and/or translation operation, as in the case of the railway wagon or the helicopter, the sudden stopping of the brake machine, for example due to extraordinary reasons, causes a recoil on the traveling mean.
When the brake machine suddenly stops, the automatic-drive safety device or negative brake intervenes to maintain tension on the cables. However, the action of retaining the tension is not coordinated with the stopping of the traveling mean.
Consequently, there is an overload on the cable, and possibly damage or breakage thereof, with obvious risks and problems of safety for the operators, in particular those on board the aerial or railway mean, and also for everything that is connected to or is under the cable: this in practice frustrates the safety function of the negative brake. It is particularly important to prevent these problems if a helicopter is used for stretching, where a recoil can lead to instability and a loss of control of the aircraft, with obvious consequences.
It is also known, in some more evolved solutions, to connect means to accumulate pressure energy of the working fluid, such as for example a hydraulic accumulator, to the negative brake.
Normally, in this case, the hydraulic circuit provides flow distributor valves, or valves to regulate the flow, of the negative brake, which determine the flow of the working fluid from the pressure energy accumulation means and from a pumping device toward the negative brake.
The distributor valves can be the type with a maintained action, or bistable, or the type with a non-maintained action, or monostable.
If there is a break-down of the device, and if the distributor valves are the maintained action type, a loss of tension occurs in the cable, since the pressure of the working fluid in the circuit is drastically reduced, whereas if the distributor valves are the non-maintained action type, there is a sudden stoppage of the negative brake with a consequent recoil effect.
In particular, when there is a failure of electrical or hydraulic parts of the stretching machine, and if the distributor valves are the non-maintained action type, the pressure of the working fluid generated by the pressure energy accumulation means is able to prevent a sudden blockage of the rotation of the capstan, with consequent elimination of the negative recoils but the maximum pressure valve, opening, would in any case cause loss of tension in the cable.
CN201.156.651Y discloses the features of the preamble of the main claims.
One purpose of the present invention is therefore to obtain a safety device for a stretching machine that prevents the occurrence of dangerous conditions during the stretching of cables when a breakdown occurs in the braking machine.
Another purpose of the present invention is to obtain a safety device for a stretching machine that allows to complete the stretching operations without generating overload situations, or instability of the device, substantially maintaining the functioning values set before the occurrence of the breakdown.
Another purpose is to perfect a control method of a cable stretching machine which allows to perform the stretching operations safely, even when parts of the device have failed.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.